Multi-wavelengths infrared laser

ABSTRACT

A long wavelength infrared laser system is disclosed where radiation from laser sources such as frequency-doubled Nd:YAG or a Cr:LiSAF is used to resonantly pump a gain medium consisting of a holmium-doped fluoride crystal having a high active ion concentration. The laser pump source has a pulse duration that may be short enough to gain switch a particular transition or long enough to allow end-pumping with high energy densities without damage. The gain material has an absorption approximately resonant with the pump source wavelength, and the dopant concentration is selected to maximize absorption strength for a given excitation. The output radiation from the laser system consists of one or more wavelengths including, in particular 3.9 nm but also other infrared wavelengths such as 1.4 μm, 2.9 μm and 3.4 μm., several of which may be produced simultaneousely from the same laser material through the mechanism of cascade transitions.

This invention is a division of Ser. No. 09/577,388, filed May 25, 2000,now U.S. Pat. No. 6,269,108, and claims priority based on U.S.Provisional Application Ser. No. 60/135,977 filed May 26, 1999.

FIELD OF THE INVENTION

This invention relates to solid state lasers, and in particular to amethod and apparatus of generating at room temperature one or morewavelengths in the infrared part of the spectrum using highconcentration Holmium-doped fluoride crystals to maximize resonant pumpabsorption.

BACKGROUND AND PRIOR ART

It is well known that the trivalent holmium ion (Ho³⁺) is capable ofproducing stimulated emission at several different wavelengths acrossthe infrared, from 0.75 to just under 4.0 μm. For the purpose ofgenerating longer wavelengths, fluoride crystals are a preferred hostfor the holmium ion because the energy levels are spaced sufficientlyapart within the different manifolds to mitigate against rapidmultiphonon non-radiative transitions which would otherwise inhibitfluorescence at wavelengths longer than about 3 μm. Thus, while the Hotransition near 2.9 μm has been made to lase in many different crystalsincluding oxides and garnets, only fluorides exhibited stimulatedemission beyond 3 μm. It is further known that because of the richenergy level structure of Ho, a multiplicity of wavelengths can begenerated through sequential transitions between intermediate levels.

One of the most interesting Ho transitions is the one near 4 μm betweenthe ⁵I₆ and ⁵I₅ levels. There are very few active ion-host crystalcombinations that have been successfully lased this far into theinfrared, and none that have demonstrated operation levels substantiallygreater than a few millijoules at or near room temperature. As will bedescribed below, stimulated emission at 3.9 μm was previously achievedby Ho:YLF but under conditions that severely limit prospects for furtherenergy and power scaling to levels that are of interest.

The main issue limiting laser action at 3.9 μm in Ho-doped crystals,including most known fluorides, is the long fluorescence lifetime of thelower ⁵I₆ laser level coupled with the self-terminating nature of the⁵I₅→⁵I₆ transition. The long ⁵I₆ lifetime—up to a few milliseconds formost fluoride materials—limits the repetition rate of the correspondinglaser transition, whereas the much shorter lifetime of the upper ⁵I₅level—typically, no more than a few 10's of microseconds, results in aneffective three-level system for the laser transition. While it is knownin the art that cooling of a three level laser medium can be used tomore easily achieve and sustain inversion, this approach is generallyconsidered unattractive for practical laser systems because of addedcomplexity and weight. It has further been recognized that analternative way to overcome an unfavorable lifetime ratio is through useof resonant pumping, whereby the upper laser level is directly excitedby a narrow band source with frequency selected or tuned to match anabsorption line that is dynamically connected to the upper level of thedesired transition. When the resonant pump source also has a very shortpulse duration (typically about 100 nanoseconds) it is said to “gainswitch” the particular transition, in much the same way Q-switching alaser oscillator produces short duration pulses.

Resonant pumping for the purpose of generating mid-infrared wavelengthsfrom activator ions in various hosts has often been employed in theprior art. For example, in the invention disclosed in U.S. Pat. No.5,200,966 to Esterowitcz and Stoneman, the ⁴I_(11/2) upper laser stateof the erbium ion was directly pumped with a pump beam at a wavelengthof about 970 nm, causing the erbium ion to produce laser emission atsubstantially 2.8 μm, corresponding to the ⁴I_(11/2)→⁴I_(15/2) lasertransition, with high efficiency at room temperature. Because high powerdiode laser arrays with wavelengths in the 950-980 nm range haverecently become more available, there have been several successfulefforts demonstrating diode pumped, power scalable cw operation fromEr-doped lasers. However, pulsed operation has been more elusive at ornear 3 μm, even under seemingly favorable resonant pumping conditions.In another example, U.S. Pat. No. 4,330,763 to Esterowitcz and Kruertaught use of resonant pumping from a laser source at 2.06 μm to achieveinversion on the ⁷F₃→⁷F₅ line at 4.1 μm from terbium-doped YLF. A largeratio of non-radiative to radiative decay rates in this gain materialdiscriminates against broad-band pumping, but allows the use ofresonant, narrow-band excitation to produce laser action.

Heretofore, Holmium-doped lasers have also been made which are capableof pulsed operation in the infrared region of the spectrum upon resonantpumping by radiation from Nd:YAG lasers with output near 1 μm. Inparticular, pulsed emission at or near 3 μm from Ho-doped garnets suchas YAG, GGG and YALO was described wherein co-doping with suitableactivator ion such as praseodymium (Pr) was utilized to allow resonantpumping near 1 μm. For example, Anton in U.S. Pat. No. 5,070,507describes a laser system wherein a Nd-doped laser operating on anon-standard line of 1.123 μm is used to pump holmium laser to produce amoderately high energy output pulse at about 3 μm. Key to the inventionby Anton was the incorporation of holmium ion with concentrations inexcess of 15% (atomic percent) and a much lower praseodymium (Pr)concentration (on the order of 0.01%). The higher Ho concentrationallowed preferential lasing on the 2.94 μm line in Ho-doped garnetcrystals upon pumping with the 1.12 μm output of a Nd:YAG laser, whereasthe Pr ion served to quench the lifetime of the lower ⁵I₇ laser level,thereby breaking the bottleneck of the normally self-terminating ⁵I₅→⁵I₆transition.

In the early demonstrations of the long wavelength transitions inHo³⁺-doped YLF using resonant pumping of the ⁵S₂ manifold with shortpulse green lasers, laser action on the 3.9 μm line was achieved as partof a sequence with other transitions, a process known in the art ascascade lasing. Specifically, using a frequency-doubled short pulse (20ns) Nd:glass laser operating at 535 nm to pump a 1% Ho:YLF crystal, thetwo-line ⁵S₂→⁵I₅, ⁵I₅→⁵I₆(1.392 μm, 3.914 μm) and ⁵S₂→⁵I₅, ⁵I₅→⁵I₇(1.392 μm, 1.673 μm) cascade transitions were successfully lased at roomtemperature (see L. Esterowitz, R. C. Eckardt and R. E. Allen, Appl.Phys. Lett., 35,236, (1979)). Three-step laser transitions, for exampleat 3.4 μm, 3.9 μm and 2.9 μm were also reported (see R. C. Eckart, L.Esterowitz and Y. P. Lee, Procs. Int'l Conf. Lasers, pp. 380 (1981)) inHo:YLF using the longer 1 μs pulse from a pulsed dye laser tuned to535.5 nm. These and similar results were further described in U.S. Pat.No. 4,321,559 to Esterowitz and Eckardt. A key feature in these earlydescriptions of resonantly pumped cascade lasing was that cascadeprocesses, whereby one laser transition sequentially pumps a lower lasertransition in the same material, could be viewed as one form of resonantself-pumping. By causing population inversion to occur sequentially,cascade laser action can therefore improve the efficiency of lasertransitions between intermediate manifolds, as well as produce radiationconsisting of two or more wavelengths. In the case of short pulse greenlaser excitation of the high lying ⁵S₂ state, cavity optics can beselected to preferentially lase a given sequence of transitions. Forexample, by using one set of coated optics, the excited ⁵S₂ statepopulation could be directly transferred to the intermediate ⁵I₅ level,which then serves as the upper level for a subsequent 3.9 μm lasertransition to the ⁵I₆ level. A different set of cavity mirrors cause thesecond lasing step to occur on the 1.7 μm ⁵I₅→⁵I₇ line.

Yet, although prior art describing the advantages of resonant pumpingand multi-wavelengths cascade lasing was related nearly two decades ago,to date no practical Ho-doped laser has been constructed with one outputwavelength near either the 2.9μ or 3.9 μm lines, using principles taughtby Esterowitcz and Eckardt. One problem with prior art systems based onresonant pumping is that they require a laser with a wavelength tunedclosely to an appropriate absorption band of the laser material. Forexample, in the case the Ho ion, lasing at 3.9 μm was previouslyobtained only as part of a sequence of cascade transitions, byresonantly pumping the ⁵I₈ ground state to the ⁵S₂, ⁵F₄ level. Toincrease the pumping efficiency, the green beam had to be tuned close tothe appropriate absorption peak, which in fluorides is near 535 nm. Thiswavelength matches up poorly with most readily available commerciallasers, which is one of the factors precluding practical application ofsuch cascade lasers to date. Similarly, the methods and system disclosedby Anthon for generating 2.9 μm radiation from Ho-doped garnets, whilerecognizing the benefits to improved efficiencies that could be obtainedby increasing holmium concentrations, still required a pump laser tunedto 1.1 μm, which is a difficult wavelength to obtain from a practicallaser system, especially if short pulse operation is desired as well.Thus, even if pump lasers with wavelengths suitable for pumping holmiumcould be constructed, other conditions on the pulse duration, energy,repetition rate, and beam quality may place additional limitations onpractical implementations of the infrared laser system with the outputpower, output wavelengths and efficiency desired.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to disclose a meansfor achieving efficient room temperature laser operation at 3.9 μm froma holmium-doped fluoride crystal pumped by a practical pulsed sourcetuned to a resonance, and with pulse duration short enough to allowpopulation inversion between the upper ⁵I₅ level and the long lived ⁵I₆lower laser level.

It is another object of the invention to disclose a pulsed Ho-dopedlaser operatively configured as a 2.9 μm or 3.9 μm downconverter for ashorter wavelength laser that is available as a commercial source.Examples of such sources include the 532 nm from frequency-doubledNd:YAG, Nd:Vanadate, or other Nd-doped systems a Ti:sapphire or Cr:LiSAFlaser tuned to about 890 nm, a fosterite or fiber Raman laser operatingnear 1.2 μm.

It is an additional object to be able to efficiently accomplish saiddownconversion utilizing short pulse (nanosecond) pump lasers, therebygain switching the transitions near 2.9 and/or 3.9 μm so as to produceshort pulses at these infrared wavelengths.

It is still another object to generate one or more different wavelengthsin the infrared between 750 nm and 4 μm, but specifically including thewavelengths near 2.9 and/or 3.9 μm, utilizing resonant pumping of aholmium-doped fluoride crystal with a shorter wavelength pump laser.

It is a further object to provide a method and system for generatingsaid output wavelengths alone or in a cascade with other infraredwavelengths at output energies scalable to over 10 millijoules and withrepetition rates scalable to over 10 Hz.

It is yet another object to disclose methods for generating energyscalable longer infrared wavelengths at room temperature using aresonant pump source with pulse duration that is sufficiently long toenable efficient pumping even from end-pumped configurations. It istherefore a special object to be able to operate the pump laser atenergy densities that are well above the threshold for sustained laseroscillation while staying below damage thresholds to sensitive IRcoatings. In various embodiments of the invention such pump sources mayinclude free running, or long pulse tunable Cr:LiSAF or Ti:sapphirelasers, frequency-doubled Nd-doped lasers, Raman fiber lasers and highpower, quasi-cw semiconductor laser arrays.

In accordance with the above objectives, system and method is disclosedfor generating at least one long infrared wavelength from aholmium-doped fluoride laser source pumped by a resonant pulsednarrow-band source. The invention includes pump sources with shortenough pulse durations to gain switch a particular transition and alsopump sources with long pulses but sufficiently high energy dsensity toovercome the saturation density associated with the transition. Ofparticular importance to the present invention are techniques forselecting the holmium concentration so as to optimize absorption at awavelength that is available as a practical commercial laser source. Inpreferred embodimets of the invention the particular wavelengths of 3.9μm and 2.9 μm are generated alone or in sequence with each other or withother wavelengths including but not limited to 1.4 μm, 2.4 μm and 2.0μm. Pump wavelengths include 532 nm, such as is available from stanfdardNd-doped lasers, 890 nm from Cr:LiSAF, Ti:sapphire, or diode laserarrays and 1.2 μm from, for example fosterite or Raman fiber lasers.

Further objects and advantages of this invention will be apparent fromthe following detailed description of a presently preferred embodiment,which is illustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an energy level diagram and a preferred embodiment ofcascade lasing in a Ho³⁺ doped fluoride material, such as Ho:BYF, using532 μm pumping.

FIG. 2 shows the absorption spectrum of 10% Ho:BYF in the 532 nmspectral range.

FIG. 3 shows the cavity layout for a preferred embodiment of gainswitched 3.9 μm Ho:BYF laser resonantly end-pumped by short pulseNd-doped 532 nm laser.

FIG. 4 shows a plot of the 3.9 μm output energy as a function of 532 nmabsorbed input energy for cascade lasing in 10% Ho:BYF pumped by aQ-switched, frequency-doubled Nd:YAG laser.

FIG. 5 shows another preferred embodiment for generating radiation at3.9 μm from Ho-doped fluoride crystal resonantly pumped at 890 nm.

FIG. 6 shows an absorption spectrum near 890 nm for (a) 10% and (b) 20%Ho:BYF.

FIG. 7 shows a schematic of a laser oscillator for generating radiationat 3.9 μm from Ho:BYF using long pulse resonant pumping at 890 nm.

FIG. 8 shows a plot of the 3.9 μm output energy as a function of 890 nmabsorbed input energy for 10% and 20% Ho:BYF pumped by a 50 μs Cr:LISAFlaser.

FIG. 9 shows several alternative options for generating radiation at 2.9μm from resonantly pumped Ho-doped fluoride laser.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

As earlier noted and to which now should be referenced, FIG. 1 portraysthe cascade process involving the ⁵S₂−⁵I₅ (1.4 μm) and ⁵I₅−⁵I₆ (3.9 μm)cascade transitions in the output of one preferred embodiment usingpulsed resonant pumping of a holmium-doped crystal such as BaY₂F₈(Ho:BYF) at a wavelength of 532 nm. This corresponds to the frequencydoubled wavelength of a common Nd-doped laser such as Nd:YAG orNd:Vanadate. Ho-doped BYF was selected as the preferred crystal over themore commonly available Ho:YLF because it has generally longerfluorescence lifetimes, which favors stimulated emission betweenintermediate levels. Other holmium-doped crystals that may be ofinterest but are not yet available commercially include NaYF₄, (NaYF)and KY₃F₁₀(KYF), as well as fluoride fibers such as ZBLAN.

For the purpose of validating the salient features of the presentinvention, key spectroscopic and dynamic characteristics were determinedfor both Ho:BYF and Ho:YLF crystals, at several concentrations between1% and 20% (atomic percent). All the crystals were grown using aconventional Czochralski technique from purified starting materials.Crystals with the general properties described herein can be acquiredcommercially, for example, from AC materials, Inc.

As a key element of the invention, higher concentrations of the activeHo ion are used principally to increase the absorption strength and toalso extend the absorption bandwidth, thereby allowing effectiveexcitation off the peak of the absorption curve. The absorption at 532nm was measured for both Ho:YLF and Ho:BYF at several concentrations.For Ho:YLF it was found to range from 0.6 cm⁻¹ for a 1% concentrationsample to 5.6 cm⁻¹ in the 20% sample. The trend is similar for Ho:BYF.An absorption spectrum for the 10% Ho:BYF sample used in subsequentexperiments is shown in FIG. 2, from which the absorption coefficient at532 nm is measured table 2.42 cm⁻¹ for polarization parallel to theb-axis. This results in more than 50% absorption for a 5 mm long rod andwould be adequate for laser end-pumped operation. For a 20% Ho:BYFsample, the 532 nm absorption was measured to be about a factor of 2higher (for polarization in the X-Z plane) than in the 10% sample, i.e.(about 4.8 cm⁻¹). Even with some uncertainty due to differingpolarizations between the two samples, the measurement confirms theexpected trend of increasing absorption with increasing Hoconcentration. It is also known that higher concentrations of the Ho ionenhance cross relaxation processes, shortening the effective lifetimesof certain levels, and increasing the efficiency of energy transferprocesses. In particular, we find evidence of a strong ⁵S₂−⁵I₄, ⁵I₈−⁵I₇cross relaxation process, as reflected in the radical decrease in theobserved fluorescent lifetime of the ⁵S₂ level—from 280 μs in 1% Ho:BYFto about 16 μs in 10% Ho and to about 3.7 μs in a 20% sample. Thelifetimes were determined by the standard 1/e method from time dependentfluorescence decay data detected with a 0.3 m monochromator, taking careto suppress any scattered light with an appropriate bandpass filter.This shortening of the ⁵S₂ lifetime would not, however, affect theefficiency of the laser transitions initiated at this level, includingthe one at 1.4 μm, as long as the pump pulse is shorter than about 3 μs.For comparison, it was noted that, in Ho:YLF, the ⁵S₂ lifetimes areconsiderably smaller than those measured in BYF (by factors of between 2and 3) with a similar trend of substantial decrease in lifetime withincreasing concentration. It is therefore expected that for gainswitched pulses, using for example 532 nm excitation with Q-switchedpulse durations of typically up to a few 10's of nanosecond, there wouldnot be a substantial difference in laser performance between Ho:YLF andHo:BYF for the cascade transitions of FIG. 1, at similar pump absorptionstrengths. However, for situations requiring longer pulse excitations,Ho:BYF offers a clear advantage, because of the longer lifetimesassociated with all the intermediate levels. Longer pump pulses may bepreferred, for example when higher energy storage is desired and/orcoating damage limitations prescribe lower incident pump peak powerdensities.

It is further noted that time resolved fluorescence decay measurementsof other intermediate levels in Ho:BYF revealed no significant lifetimequenching of either the ⁵I₆ or ⁵I₇ levels and only slight effect on thelifetime of the ⁵I₅ level (less than 10% decrease) even for 20% Hoconcentrations. Therefore the dynamic behavior of the 3.9 μm lasertransition should directly reflect the behavior of the precedingtransition from ⁵S₂. The weak dependence of the intermediate ⁵I₆ and ⁵I₇level lifetimes on the holmium concentration is similar to thatreferenced in the prior art invention to Anton (U.S. Pat. No. 5,070,507)for holmium-doped garnets such as YAG and GGG, except that thecorresponding lifetimes in fluorides are generally higher. However,although use of high concentration Ho for the purpose of increasingabsorption of a pump source was taught by Anton, this was done only inreference to pumping the ⁵I₆ level with a Nd:YAG laser modified to emitnear 1.1 μm, and demonstrating laser action on the 2.9 μm line byrelying on energy transfer to a co-dopant such as Pr³⁺ to suppress the⁵I₇ lifetime and achieve inversion. More specifically, the measurementsby Anton showed that incorporation of low concentrations of Pr could beused to eliminate self termination of the ⁵I₆→⁵I₇ transition near 3 μmin Ho-doped garnets. It was not however realized by Anton that higherconcentrations of Ho can also have the very beneficial use of enhancingabsorption to a preferred level and allowing access to practicalwavelengths available from standard commercial sources. Neither was itunderstood by Anton that by taking advantage of cascade transitions,further flexibility in selecting and tuning the pump wavelength can berealized, thereby giving rise to a multiplicity of wavelengths that canall be generated from a single Ho-doped material. It should be furtherpointed out that the present invention does not rely on a co-dopant, asthe preferred embodiment employs pulsed pumping with short pulsedurations which circumvent self-termination of the transitions ofinterest, including those shown in FIG. 1 as well as alternativetransitions, such as the ⁵I₆→⁵I₇ near 2.9 that will be described furtherbelow. As will also be described below, in another embodiment, longpulse pumping is employed but laser action is sustained by employingsufficiently high power densities. The possibility of applying suchprinciples to a three-level laser transition was clearly not anticipatedby Anton. The present invention does not, however, preclude use of aco-dopant for the purpose of selectively quenching lower level lifetimesand permitting scaling of the laser repetition rates, as long asconventional, commercially available, pump laser sources can be utilizedto provide the excitation. Therefore, a common element to all thepreferred embodiments of this invention, is that they result in a highlypractical and cost effective infrared laser system that is also energyand power scalable.

Referring now to FIG. 3, a cavity layout for a 4 μm Ho:BYF laser isportrayed with resonant pumping provided by a laser source 10,comprising in a preferred embodiment, a Q-switched, frequency-doubledNd:YAG laser at 532 μm. In this example, the laser cavity 12 consists ofa 1 m radius of curvature input mirror 14, HR coated for reflection at1.4 and 3.9 μm, and a flat output coupler 16 coated for high reflectionat 1.4 μm and about 94% reflection at 3.9 μm. The 6 mm long (uncoated)Ho:BYF crystal 18 is placed near the output coupler 16. The gainmaterial 18 was end-pumped with 20 nsec long pulses at a repetition rateof 10 Hz. Also shown in FIG. 3 is a half-wave plate/polarizercombination 20 which could be used to continuously vary the pump energy,and a second halfwave plate 22 which allows optimal alignment of thepump polarization with respect to the crystal axis. A 3 mm long passfilter 24 was used to isolate the 3.9 μm radiation.

Lasing at 1.4 μm for the foregoing example was readily achieved with alow threshold of less than 1 mJ absorbed pump energy. Threshold forcascade lasing at 3.9 μm was reached at approximately 5 mJ of absorbed(10 mJ incident) pump energy at 532 μm. FIG. 4 shows a plot of themeasured output energy at 3.9 μm as a function of absorbed 532 nm pumpenergy. As the plot indicates, a maximum output energy of 2.6 mJ wasachieved at an absorbed pump energy of 30 mJ. This corresponds to aslope efficiency of 10.4% or a quantum efficiency of ˜76.5%, which isnear theoretical values.

These results are remarkable, given that the laser cavity of FIG. 3 doesnot represent an optimal arrangement, as neither the infrared coatingsnor mode matching were fully optimized. Also since the crystal was notcoated, Fresnel losses lowered the efficiency. It should therefore berealized that better, higher performance optics and coatings as well asfabrication of longer crystals to obtain still more pump absorptionwould further improve the overall efficiency of the infrared laser. Agreen laser pump source with longer pulses (preferably 100's ofnanoseconds to a few microseconds) would be especially advantageous inreducing the risk of damage to the mirror coatings and allowing scalingto much higher output energies from end-pumped cavity designs.Principles of the method and system of this embodiment could thereforelead to a practical, energy scalable, short pulse laser at IRwavelengths not available from any other directly emitting solid statelaser. The laser can be construed as a downconverter for standard pulsedgreen lasers, with an energy scaling potential limited primarily bycoating damage.

The principal limitation of the laser of FIG. 1 is that repetition ratesare constrained by the lifetime of the ⁵I₆ level to less than about 100Hz. It may, however, be possible to utilize a suitable co-dopant toquench the lower level lifetime, thereby allowing correspondingextention of the repetition rate. It should threrefore be understoodthat inclusion of co-dopant for the purpose of scaling the repetitionrates of a 3.9 μm laser falls within the scope of the present invention.A further advantage of the system of FIG. 1 is that laser designs can beconstructed that also provide for efficient laser action at 1.4 μm, awavelength which has several important applications including medicaltelecommunications and in eyesafe ranging and laser radar systems.Although the laser oscillator of FIG. 3 was not optimized for thiswavelength, we estimate that slope efficiencies of up to 30% arefeasible, making the embodiment of FIG. 1 an attractive option forgenerating this unique wavelength.

Many alternative designs of the 532 nm pumped 3.9 μm laser and/or 1.4 μmlaser are possible, and fall under the scope of the present invention.These include alternative output coupling optics, using for example adichroic prism to separate the two output wavelengths and side pumpedconfigurations based on cavity designs that are known in the art.Although the preferred embodiment is described by reference to afrequency-doubled Nd:YAG as the pump laser, this should not be construedas limiting the domain of applicability of the invention. In particular,a number of other pulsed green laser sources can be advantageouselyutilized to provide the pump radiation, including, but not limited to,frequency-doubled Nd:YALO and Nd:YVO₄. Further increases in the Hoconcentration to beyond 20% are also feasible, and will likely result instill greater absorption. There is however a trade-off against the ⁵S₂lifetime which places an upper limit on desirable Ho concentration for agiven pump pulse duration, and such trade-offs should be taken intoaccount in designing a practical laser system based on highconcentration Ho doped fluoride materials.

While the cascade process depicted in FIG. 1 represents an attractiveoption for generating 3.9 μm radiation, some losses due to competingnon-radiative decay channels and possible cross relaxation processescannot be avoided, especially when materials with very high Hoconcentration are utilized. The most efficient method for lasing the 3.9μm transition is therefore to populate the ⁵I₅ level directly withoutexciting the higher lying levels. Accordingly, there is shown in FIG. 5another preferred embodiment of the invention, whereby radiation tunedto approximately 890 nm corresponding to the ⁵I₈→⁵I₅ excitation is usedto directly pump the upper level of the ⁵I₅→⁵I₆ transition. This pumpwavelength is attractive because it corresponds to existing tunablesolid state lasers such as Ti:sapphire and Cr:LiSAF, and also matchesthe output available from high power laser diode arrays. The primarylimitation to this approach is the relatively low absorption crosssection near the 890 nm transition. However, by going to increasinglyhigher Ho concentrations, the absorption length can be considerablyincreased. To illustrate the available absorption strength near 890 nm,FIG. 6 shows the absorption spectrum for (a) 10% and (b) 20% Ho:BYF inthis spectral region. The absorption length derived from thesemeasurements is about 0.5 cm⁻¹ for the 10% Ho, increasing to about 1cm⁻¹ for the 20% doped material. Thus , for a 1 cm long 20% Ho:BYFcrystal, over 67% of the incident light intensity at 890 mn is absorbed.

In a preferred embodiment corresponding to the scheme depicted in FIG.5, the 890 nm excitation source has a long pulse but is capable ofdelivering enough energy during the pulse to exceed the saturationdensity of the gain material by a substantial factor.

As long as the repetition rate is smaller than the inverse of the lowerstate lifetime, lasting conditions can be realized such that the upperlaser level is directly and continuousely populated to achieve andmaintain inversion throughout the pump pulse duration.

Thereby achieving and maintaining inversion throughout the pump pulseduration. In this manner of operation, stimulated emission can becreated and sustained even from levels lacking sufficiently longfluorescence lifetimes relative to a lower laser level. These areimportant considerations for situations when long pulse pumping isdesired as a way to lower the incident peak power thereby reducing therisk of optical damage to coatings. Peak power damage thresholds areknown to be smaller for mid-infrared coatings and damage can become anespecially significant issue in end-pumped configurations, wheresensitive dichroic coatings are typically employed. On the other hand,longer pump pulse durations reduce the available peak power, so the longpulse pump source must be capable of delivering enough energy per pulseto overcome the threshold as (as defined by saturation power density) byat least a factor of 5 to 10, thus sustaining laser operation. Sincesaturation power density is inversely proportional to the levellifetime, and the ⁵I₆ lifetime is relatively short—only about 40 μs in10% Ho:BYF—the saturation power density for the ⁵I₅→⁵I₆ transition isestimated to be as high as 100 kW/cm². Therefore narrow-band pumpingwith a long (10's of microseconds) pulse require a source scalable tocorrespondingly high energies. Flashlamp-pumped Cr:liSAF is one suchsource, as it can deliver well over 0.5 J with a beam quality that isgood enough to allow focusing to small spot, thus achieving therequisite power densities within the gain material.

FIG. 7 is a schematic for a 3.9 μm Ho:BYF laser end-pumped at awavelength of 890 nm. As shown in FIG. 7, the pump laser 40 comprises,in a preferred embodiment, a Cr:LiSAF laser which is operated in a freerunning mode with a pulse duration of approximately 50 μs. In this mode,the pulse format typically consists of a series of relaxationoscillations, with beam quality that is several times the diffractionlimit. Also shown in FIG. 7 are optics 42 to couple and focus the 890 nmradiation into the Ho:BYF rod 44, which in this illustrative example wasapproximately 2 cm long, giving about 67% absorption. On one input end,the Ho:BYF crystal coating 45 is applied to transmit the 890 nm beamwhile providing high reflection at 3.9 μm. The rod on this end had aweak curvature of about 1 m in this example. An output coupler 46 isalso shown which preferably has a reflectivity typically between 95 and99% at 3.9 μm. A longpass filter 48 is placed at the output end tosuppress radiation below 3.2 μm. The output energy at 3.9 μm wasmeasured with a fast HCT detector.

FIG. 8 shows plots of the measured output energy at 3.9 μm as a functionof absorbed 890 nm pump energy for the 10% at two output mirrorreflectivities and for the two polarizations of the 20% Ho:BYF, each at95% reflectivity. As the Figure shows, output energies of over 20 mJwere measured for the a 20% Ho:BYF sample, corresponding to 4.7% slopeefficiency. It is noted that the HR coatings used in this experimentwere of poor quality. It is projected that better quality coatings andmore optimized cavity designs could produce 3.9 μm energies in excess of50 mJ and slope efficencies approaching 10%.

In alternative embodiments of the system of FIG. 5 other 890 nm pumplasers could be utilized including other Cr-doped lasers, Ti:sapphire(lamp or laser-pumped), or high power diode laser arrays. Of these, thelatter is highly attractive because of the high efficiencies availablefrom semiconductor lasers. However, diode laser arrays with high powerdensities (approaching 1 kW/cm2) would be required, assuming pulsedurations on the order of 100 ms. Diodes with higher brightness and moreoutput power are expected to become commercially available in thenear-term making diode pumping of the 3.9 μm transition feasible.

FIG. 9 shows a number of options for producing radiation at 2.9 μm fromHo-doped fluorides using the principles described in this invention. Forexample, cavity coatings can be selected to generate 2.9 μm laser pulsesas the third wavelength in the 1.4 and 3.9 mm cascade sequence producedby pumping the ⁵S₂ level at or mean 532 nm. Other coatings can be usedto produce alternative cascades, including the three line transitions at2.37, 3.9 and and 2.9 μm. Still different optics would allow radiationat 2.9 μm to be produced directly using, for example, the same 890 nmlong pulse pump employed in demonstrating 3.9 μm lasing. Alternatively a1.2 μm pump could be employed to directly excite the ⁵I₆ level.Available lasers at this wavelength include Raman fiber lasers (whichcan be tuned to a resonance and are now becoming available with highpowers and brightness) tunable fosterite laser, which may be tuned to aclose resonance with the absorption peak mean 1.2 μm.

While the invention has been described, discosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

We claim:
 1. A laser source for generating at least one infraredwavelength, comprising in combination: a pump source having a wavelengthand a peak power density that overcomes a threshold of a selected lasertransition; means for coupling radiation from the pump source into alaser cavity; a gain material of holmium-doped fluoride within the lasercavity having an adjustable dopant concentration of holmium to maximizeabsorption of the pump source wavelength; and optic means for generatingat least one wavelength between approximately 1.4 μm and approximately 4μm from said source.
 2. The laser source of claim 1, wherein the opticmeans is configured to generate at least two wavelengths.
 3. The lasersource of claim 2, wherein the two wavelengths include: approximately1.4 μm and approximately 3.9 μm.
 4. The laser source of claim 2, whereinthe two wavelengths include: approximately 2.9 μm and approximately 3.9μm.
 5. The laser source of claim 1, wherein the dopant concentrationincludes: greater than 2 up to to approximately 10 atomic percentholmium.
 6. The laser source of claim 1, wherein the dopantconcentration includes: approximately 10 to approximately 20 atomicpercent holmium.
 7. The laser source of claim 1, wherein the dopantconcentration includes: greater than approximately 20 atomic percentholmium.
 8. The laser source of claim 1, wherein the pump sourceincludes: a narrow band source emitting at approximately 532 nm.
 9. Thelaser source of claim 1, wherein the pump source is: a frequency-doubledNd:YAG laser.
 10. The laser source of claim 1, wherein the pump sourceincludes: a narrow band source emitting at approximately 890 nm.
 11. Thelaser source of claim 1, wherein the pump source is: a Cr-doped laser.12. The laser source of claim 1, wherein the pump source is: a Ti dopedlaser.
 13. The laser source of claim 1, wherein the pump source ischosen from one of: a diode laser and a diode laser array.
 14. The lasersource of claim 1, wherein the pump source includes: a narrow bandsource emitting at approximately 1.2 microns.
 15. The laser source ofclaim 1, wherein the pump source includes: a fiber laser.
 16. The lasersource of claim 1, wherein the optic means is configured to generate awavelength of: approximately 2.9 μm.
 17. The laser source of claim 1,wherein the.optic means generate a wavelength of: approximately 3.9 μm.18. The laser source of claim 1, wherein the pump pulse duration allowsa switch of said laser transition.
 19. The laser source of claim 1,wherein the pump pulse duration is allows operation at high energydensities.
 20. The gain material of claim 1, wherein the homium-dopedfluoride is selected from one of: Ho:BYF, Ho:YLF, Ho:NaYF and Ho:KYF.